I started driving for a living at 17,but had to wait till I was 21 to get my first LTL Teamsters city job.
Which was 1966,I logged in 45 years,and have been retired for a few years.
But when we were still Overnite,our dock crew were folks that started part-time at 18,I trained them to drive Semi's,and hook sets in the yard only.
They all had to wait till they turned 21 to start driving for the company.
then they had to log in at least one year in the city before they could go for any line haul bids.
I'm not sure if UPS follows the same age requirements?